Acid-hardening self-hardening molds are each produced by adding, to refractory particles such as silica sand, a binder for mold formation containing an acid-hardening resin, and a hardening agent including phosphoric acid, an organic sulfonic acid, sulfuric acid or some other acid, mixing and kneading these components, filling the resultant kneaded sand into an original pattern such as a wooden pattern, and then hardening the acid-hardening resin. As the acid-hardening resin, a furan resin, a phenolic resin or the like is used. As the furan resin, the following is used: furfuryl alcohol, furfuryl alcohol/urea-formaldehyde resin, furfuryl alcohol/formaldehyde resin, furfuryl alcohol/phenol/formaldehyde resin, some other known modified furan resins or the like.
An important requirement for producing a mold is the mold-producing performance. In order to raise, in a self-hardening mold, the production performance of the mold, it is necessary to shorten a period required from a time when kneaded sand is filled into an original pattern to a time when the mold is stripped from the original pattern (strip time) by raising the hardening rate of the self-hardening mold after the filling.
Disclosed is, for example, a binder composition containing a hydrolyzable tannin and an aromatic aldehyde from the viewpoint of mold-producing performance (PTL 1).
Disclosed is also a binder composition containing a hydrolyzable tannin and a specific silane coupling agent from the viewpoints of storage stability as well as mold-producing performance (PTL 2).
Disclosed is also a method for producing a self-hardening resin for a mold by use of a furan-modified urea resin in order to decrease formaldehyde odor and give excellent storage stability and strength properties (PTL 3).
In the meantime, it is disclosed that, for example, 5-hydroxymethylfurfural as a compound alternative for furfuryl alcohol can decrease harmful matters or gases for a binder composition which does not substantially contain phenol, formaldehyde nor nitrogen (it means any amine-containing component such as urea), thereby improving the working environment (PTL 4).